About Fastener Materials

General

Fasteners are manufactured in a wide range of materials
from common steel to titanium, plastic and other exotic materials.
Many materials are further separated into different grades to describe specific alloy mixtures, hardening processes,
etc. In addition, some materials are available with a variety
of coatings or platings to enhance the corrosion resistance
or alter the appearance of the fastener.

Fastener material can be important when choosing a fastener
due to differences between materials in strength, brittleness,
corrosion resistance, galvanic corrosion properties and, of
course, cost.

When replacing fasteners, it is generally best to match what
you are replacing. Replacing a bolt with a stronger one is
not always safe. Harder bolts tend to be more brittle and
may fail in specific applications. Also some equipment is
designed so that the bolts will fail before more expensive
or critical items are damaged. In some environments, such as
salt water, galvanic corrosion must also be considered if changing
fastener materials. For more information see our About
Galvanic Corrosion page.

Materials

Stainless Steel

Stainless steel is an alloy of low carbon steel and chromium for enhanced corrosion characteristics. Stainless steel is highly corrosion resistant for the price. Because the anti-corrosive properties are inherent to the metal, it will not lose this resistance if scratched during installation or use.

It is a common misconception that stainless steel is stronger than regular steel. In fact, due to their low carbon content, many stainless steel alloys cannot be hardened through heat treatment. Therefore, when compared to regular steel, the stainless alloys used in bolts are slightly stronger than an un-hardened (grade 2) steel but significantly weaker than hardened steel fasteners.

Most stainless steel fasteners are much less magnetic than regular steel fasteners though some grades will be slightly magnetic.

18-8 Stainless

18-8 refers to any stainless steel containing approximately 18% chromium and 8% nickel. This is the most common stainless designation for hardware. For information on 18-8 stainless steel material properties see our
Material
Grade Identification and Properties Chart.

Stainless 316

A highly corrosion resistant grade of stainless steel. Ideal in salt water and chlorine environments. More expensive than 18-8.

Stainless 410

A stainless alloy that is harder than 18-8 stainless steel, but not as resistant to corrosion.

Steel

Steel is the most common fastener material. Steel fasteners
are available plain as well as with various surface treatments
such as zinc plating, galvanization, and chrome plating.

Steel fasteners are commonly available in 4 grades: Grade 2, Grade 5, Grade 8, and Alloy Steel. Many
other grades exist but are used far less often. Grade
2, 5, and 8 are usually plated with a slightly blue-ish or yellow zinc
coating, or are galvanized, to resist corrosion.

Determining Bolt Grade

Bolts are typically marked on the head to show
what grade bolt they are. For a list of the most common
grade markings see our Material
Grade Identification and Properties Chart. Note that, in addition to the grade marking, many bolts also have a manufacturer's mark.

Grade 2

Grade 2 is a standard hardware grade steel. This is the
most common grade of steel fastener and is the least expensive.
Except a possible manufacturer's mark, Grade 2 bolts have no head marking.

Grade 5 / Grade F

Grade 5 bolts are hardened to increase strength and are
the most common bolts found in automotive applications.
Grade 5 bolts have 3 evenly spaced radial lines on the head.

Grade 8 / Grade G

Grade 8 bolts have been hardened more than grade 5 bolts.
Thus they are stronger and are used in demanding applications
such as automotive suspensions. Grade 8 bolts have 6 evenly
spaced radial lines on the head.

Alloy Steel

Alloy steel bolts are made from a high strength steel alloy
and are further heat treated. Alloy steel bolts are typically
not plated, resulting in a dull black finish. Alloy steel
bolts are extremely strong but very brittle.

Silicon Bronze

Silicon bronze, often referred to simply as bronze, is an
alloy made mostly of copper and tin with a small amount of
silicon. Bronze is used primarily in marine environments.
It is preferred over stainless in wooden boat construction
and re-fastening due to its superior corrosion resistance,
and over brass due to its higher strength. Bronze is similar
to copper in color and is also sometimes seen in fine woodworking
where it is used for its appearance. The main drawback of
bronze is its high cost.

Brass

Brass is an alloy of primarily copper and zinc. Brass is highly
corrosion resistant and electrically conductive. However,
its use as a fastener is somewhat limited due to its relative
softness. It is used primarily for its appearance.

Aluminum

Aluminum is a light, soft, corrosion resistant metal. Like stainless steel, aluminum's corrosion resistance is inherent to the material. Therefore, scratches and nicks will not effect the corrosion resistance.

Fasteners are made from a variety of aluminum alloys, with elements such as manganese, silicon, iron, magnesium, zinc, copper, and silicon being added to increase strength and melting point.

Rivets are often made from aluminum alloys in the 5000-series, which uses magnesium as the primary alloying element.

Coatings

Zinc Plating

Many steel fasteners are electroplated with zinc for better
corrosion resistance. Fasteners that have been zinc plated
have a shiny, silvery or golden appearance, referred to as clear
or yellow zinc respectively. They are fairly corrosion resistant
but will rust if the coating is destroyed or if exposed to
a marine environment.

Hot Dip Galvanizing

Galvanizing is another coating involving the application of
a layer of zinc. Hot dip galvanizing puts the thickest
possible coating on the metal, resulting in superior corrosion
resistance. Due to the thickness of the coating hot dipped
galvanized bolts are not compatible with other nuts. Galvanized
nuts are tapped slightly larger than other nuts to accommodate
this coating.
Hot dipped galvanized fasteners are frequently used outdoors, especially in coastal
environments.

Chrome

Fasteners are chrome plated and polished for appearance. Chrome plating
provides similar corrosion resistance to zinc plating. The
main drawback of polished chrome is its high cost. If more
corrosion resistance is required, stainless steel may be chrome
plated, preventing any corrosion should the chrome be penetrated.